(a) Field of the Invention
The present invention relates to an active matrix substrate including a substrate made of a plastic material, an apparatus for manufacturing the same and a display device using the same.
(b) Description of Related Art
Active matrix substrates including a plurality of thin film transistors (hereinafter abbreviated as TFT) formed on a glass substrate have been widely used for display devices such as liquid crystal displays, organic EL displays and electrophoretic displays.
In recent years, such various displays have been required to have portability and flexibility. Aiming at lightweight and flexibility, research and development have been carried out on active matrix substrates using a plastic substrate in place of the glass substrate.
The plastic substrate is preferably transferred on a so-called roll-to-roll system in the course of manufacture of the active matrix substrate. Application of the roll-to-roll system is significantly advantageous in that the substrate is transferred more quickly than a sheet-by-sheet system and a batch system which have commonly been employed for the transfer of the glass substrate. In addition, the plastic substrate is continuously unwound from the roll, thereby allowing the manufacture of the active matrix substrate with significantly improved efficiency.
Because of their extremely fine structure, the TFTs are susceptible to static charge generated in the course of manufacture or after assembly into a final product. That is, if the back surface of the active matrix substrate is electrostatically charged during the manufacture, the plastic substrate (or the glass substrate) plays a roll as a capacitor to induce charge on the TFTs formed on the front surface of the active matrix substrate. This brings about a problem of electrostatic breakdown in the TFTs.
To cope with the problem, there is a known technique of forming an ITO film as a conductive film on the back surface of the glass substrate having the TFTs on the front surface thereof (e.g., see Japanese Unexamined Patent Publication No. HEI4-273166). With this configuration, static charge generated on the back surface of the glass substrate is dispersed over the entire surface of the ITO film to prevent electrostatic breakdown in the TFTs. However, in most cases, the quantity of static charge generated on the glass substrate is not so large. Therefore, this configuration has not been employed in combination with the common transfer systems for the glass substrate.
As compared with the glass substrate, the plastic substrate is usually more prone to absorb moisture and has a higher thermal expansion coefficient. Accordingly, the plastic substrate is apt to cause expansion, contraction and warpage. Therefore, in order to form the TFTs with great precision in the manufacture of the active matrix substrate, it is necessary to suppress the expansion, contraction and warpage of the plastic substrate in some way during its transfer between processing apparatuses or within a processing apparatus. In recent years, the size of the substrate has been on the increase, and therefore the problem of the substrate expansion, contraction and warpage is becoming more noticeable.
Consequently, there has been used a transfer jig which is specially made for transferring the plastic substrate while suppressing the warpage thereof (e.g., see Japanese Unexamined Patent Publication No. HEI8-86993). The transfer jig includes a support substrate having relatively high rigidity so as not to become deformed easily and an adhesive layer formed on the support substrate. The transfer jig is so configured that a substrate which is relatively low in strength and rigidity, such as a plastic substrate, is fixed onto the support substrate via the adhesive layer. In this way, the transfer jig forcibly suppresses the warpage of the plastic substrate.
If the transfer jig is made of glass, however, the transfer jig would weigh more than three times as much as the plastic substrate itself. Therefore, in reality, it is difficult to use such a transfer jig directly in combination with a common manufacturing apparatus. The use of the transfer jig requires a modification to the manufacturing apparatus itself, thereby involving a cost increase due to the modification. Further, the use of the adhesive layer in the transfer jig has a problem in that unwanted gas is generated from the adhesive layer in a vacuum or peeling of the plastic substrate off the support substrate will be difficult if the adhesive layer is cured at high temperatures. Therefore, the transfer jig as described above has been used only in the atmospheric air at relatively low temperature.
As compared with the glass substrate, the plastic substrate is more prone to cause static buildup by friction and less likely to release the built-up static charge. Therefore, if the roll-to-roll system which offers significant advantages to the plastic substrate is employed, an extremely large quantity of static charge is generated upon winding and unwinding the substrate, resulting in a significant decrease in percentage of conforming products to the obtained display devices. Moreover, if a display device including the TFTs is operated for a long time in the dried condition, a trace quantity of charge is generated on the plastic substrate by air friction. The charge continuously applies unwanted bias to the display device, resulting in imprecise display on the display device. Thus, such a long-term static buildup has been a cause of defects.